In membrane-type fluid flow transfer devices, such as hemodialyzers, one liquid flows past one surface of a semipermeable membrane, another liquid flows past the other surface, and transport of chemicals through the membrane occurs along with ultrafiltration, i.e., the passage of liquid from the side of the membrane with liquid at a higher pressure to the side with liquid at a lower pressure. In such devices it is often desirable to control the ultrafiltration, to achieve, for example, a particular patient weight at the end of a dialysis session in which some liquid passes from the blood to the dialysate.
A past method for removing a target amount of ultrafiltration during a dialysis session involved employing two or three positive displacement (generally piston-type) pumps whose pumping rates are such that the pumps pull from the dialyzer more dialysate than is pumped into it during the session (e.g., Rollo et al. U.S. Pat. No. 4,113,614, Lipps et al. U.S. Pat. No. 4,209,391 and Willock U.S. Pat. No. 4,172,033). Another method involves periodically, manually interrupting the dialysate flow to the dialyzer, measuring the rate of ultrafiltration, reading the dialysate and blood pressures to determine the transmembrane pressure (TMP; i.e., the difference of the pressures of the liquids on the two sides of the membrane) associated with the measured ultrafiltration rate, and adjusting the dialysate pressure to a value intended to achieve the total ultrafiltrate removal objective. (An example of a system that can be used in this method is disclosed in Boag et al. U.S. Pat. No. 3,990,973.) A third method to remove a target amount of ultrafiltration is disclosed in Afflerbaugh et al. U.S. Pat. No. 4,202,764; this method involves determining the TMP necessary to achieve a desired ultrafiltration rate, and then maintaining that ultrafiltration rate by maintaining the TMP.